Vibrating beam force transducers are often used as force-to-frequency converters in accelerometers, pressure sensors and related instruments. In one well-known design, described in U.S. Pat. No. 4,372,173, the force transducer is in the form of a double-ended tuning fork fabricated from crystalline quartz. The transducer comprises a pair of side-by-side beams that are connected to common mounting structures at their ends. Electrodes are deposited on the beams in predetermined patterns, and the electrodes are connected to a drive circuit. The drive circuit provides a periodic voltage that causes the beams to vibrate toward and away from one another, 180.degree. out of phase. In effect, the drive circuit and beams form an oscillator, with the beams playing the role of the frequency control crystal, i.e., a mechanical resonance of the beams controls the oscillation frequency. A tension force applied along the beams increases the resonant oscillation frequency. The frequency of the drive signal is thereby a measure of the force applied axially along the beams.
Vibrating beam force transducers require materials with low internal damping, to achieve high Q values that result in low drive power, low self-heating, and insensitivity to electronic component variations. Transducer materials for high-accuracy instruments also require extreme mechanical stability over extended cycles at high stress levels. One of the key problems in producing such transducers involves the drive and position pick-off measurement. Crystalline quartz is the most commonly used material for mechanical transducers because of its piezoelectric properties, which properties provide the ability to drive and sense mechanical motion through the use of a simple surface electrode pattern.
With the advent of low cost, micromachined mechanical structures fabricated from crystalline silicon, it has become desirable to create silicon vibrating beam transducers. However, silicon does not possess piezoelectric properties for driving and sensing beam vibration. It is therefore desirable to provide a method of exciting and sensing the resonance of a silicon beam, without adding substantial costs, mechanical instabilities, or excessive complexity. One prior approach to this problem has been to apply a piezoelectric material (e.g., zinc oxide) to a silicon beam. This approach can provide the required drive/pick-off capability, but adds complexity, instability, and thermal expansion mismatch, and tends to degrade the reliability of the sensor. Doping and thermal drive techniques can also be used, but they create significant self-heating problems, and do not provide means for sensing beam position.